Call java program (with arguments) from Python and return results back to Python

Question

I had to execute a java file(passing a single argument) from Python and after execution, return the result back to the script that called it. I wrote a little java test program that takes an argument and prints out some stuff to the stdout. Here is the java program

import java.util.*;
class TestX
{
       public static void main(String args[])
       {
            String someString = "Your input";
            System.out.println(someString + " " + args[0]);
            int resultOfSomething = 45;
            String someOutput = "Program's output is " + resultOfSomething;
            System.out.println(someOutput);
       }
}

And here is the Python script that calls the java program

import os.path,subprocess
from subprocess import STDOUT,PIPE

def compile_java(java_file):
    subprocess.check_call(['javac', java_file])

def execute_java(java_file):
    java_class,ext = os.path.splitext(java_file)
    cmd = ['java', java_class]
    proc = subprocess.Popen(cmd, stdout=PIPE, stderr=STDOUT)
    stdout,stderr = proc.communicate(input='SomeInputstring')
    print ('This was "' + stdout + '"')

compile_java('TestX.java')
execute_java('TestX.java')

I should mention that I am not familiar with java. I compiled the little java program by this command javac TestX.java. That created a class file. And when I run the java program from the command line using java TestX SomeTestString the tiny java program works as expected and outputs some text to the console.

Now the problem; When I run the same java program from Python using the code above - 1) There is an error

This was "Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 0

        at TestX.main(TestX.java:7)
"

When I comment out the line print ('This was "' + stdout + '"') in the Python program, then the Python program runs and seems to execute the java program, but nothing is returned to the console. There is something extremely simple that I am missing here or it's just a silly mistake that I made. Any pointers? Thanks.

I tried a few different methods to output the result, like by using printf, ,and even using this class StdOut. All of them seem to behave the same.

If I were to explain this in a nutshell, it's this; My Python script should call a java program with a single argument. The java program takes in the input, performs certain functions and returns a result which I should be able to read from the Python program that called it.

Answer 1

Command line arguments are not stdin. args[0] is a command line argument. You should be able to do it like

cmd = ['java', java_class, 'SomeInputString']